There are at least four forms of crystalline carbon. Graphite is the most common, and diamond is the other well–known one. But "under some circumstances" (according to Wikipedia) carbon crystallizes to lonsdaleite; and any quizzer worth his or her salt should have heard of the fullerenes, which include buckminsterfullerite (a.k.a. 'bucky balls').
The fullerenes are synthetic, so they could have been ruled out by the inclusion of the phrase "naturally–occurring" in the question. But "lonsdaleite" would appear to be a perfectly acceptable – if obscure – alternative answer.
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